Conventionally, two types of devices, namely a thermionic cathode device and cold cathode device, are known as electron-emitting devices. Known examples of the cold cathode devices are surface-conduction type emitting devices, field emission type emitting devices (to be referred to as FE type emitting devices hereinafter), and metal/insulator/metal type emitting devices (to be referred to as MIM type emitting devices hereinafter).
As surface-conduction type emitting devices, e.g., M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965) and other examples (to be described later) are known.
The surface-conduction type emitting device utilizes the phenomenon that electrons are emitted by flowing a current through a small-area thin film formed on a substrate in parallel with the film surface. The surface-conduction type emitting device includes an emitting device using an Au thin film [G. Dittmer, “Thin Solid Films”, 9, 317 (1972)], an emitting device using an In2O3/SnO2 thin film [M. Hartwell and C. G. Fonstad, “IEEE Trans. ED Conf.”, 519 (1975)], an emitting device using a carbon thin film [Hisashi Araki et al., “Vacuum”, Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an emitting device using an SnO2 thin film by Elinson et al.
FIG. 28 is a plan view showing the device by M. Hartwell et al. described above as a typical example of the device structures of these surface-conduction type emitting devices. In FIG. 28, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. The conductive thin film 3004 has an H-shaped flat pattern, as shown in FIG. 28. The conductive thin film 3004 undergoes electrification processing (to be referred to as forming processing), thereby forming an electron-emitting portion 3005. An interval L in FIG. 28 is set to 0.5 to 1 [mm], and W is set to 0.1 [mm]. The electron-emitting portion 3005 is illustrated in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience. However, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface-conduction type emitting devices by M. Hartwell et al. and the like, typically the electron-emitting portion 3005 is formed by performing electrification processing called forming processing for the conductive thin film 3004 before electron emission. In electrification forming, a constant DC voltage or a DC voltage which rises at a very low rate of, e.g., about 1 V/min is applied across the conductive thin film 3004 to locally destroy, deform or denature the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the locally destroyed, deformed or denatured part of the conductive thin film 3004 has a fissure. When an appropriate voltage is applied to the conductive thin film 3004 after electrification forming, electrons are emitted near the fissure.
Known examples of the FE type devices are described in W. P. Dyke and W. W. Dolan, “Field emission”, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, “Physical properties of thin-film field emission cathodes with molybdenium cones”, J. Appl. Phys., 47, 5248 (1976).
FIG. 29 is a sectional view showing the device by C. A. Spindt et al. described above as a typical example of the FE type device structure. In FIG. 29, reference numeral 3010 denotes a substrate; 3011, an emitter wiring line made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. This device is caused to produce a field emission from the tip of the emitter cone 3012 by applying an appropriate voltage between the emitter cone 3012 and the gate electrode 3014.
As another FE type device structure, there is an example in which an emitter and gate electrode are arranged on a substrate to be almost parallel to the substrate surface, instead of the multilayered structure of FIG. 29.
A known example of the MIM type emitting devices is described in C. A. Mead, “Operation of tunnel-emission Devices, J. Appl. Phys., 32, 646 (1961).
FIG. 30 shows a typical example of the MIM type device structure. FIG. 30 is a sectional view. Reference numeral 3020 denotes a substrate; 3021, a lower metal electrode; 3022, a thin insulating layer having a thickness of about 100 Å; and 3023, an upper metal electrode having a thickness of about 80 to 300 Å. The MIM type emitting device emits electrons from the surface of the upper electrode 3023 by applying an appropriate voltage between the upper electrode 3023 and the lower electrode 3021.
Since these cold cathode devices can emit electrons at a lower temperature, compared to the thermionic cathode devices, the cold cathode devices do not require any heater. The cold cathode device has a structure simpler than that of the thermionic cathode device, and it is possible to fabricate elements that are finer. Even if many devices are arranged on a substrate at a high density, problems such as heat fusion of the substrate hardly arise. In addition, the response speed of the cold cathode device is high, while the response speed of the thermionic cathode device is low because it operates upon heating by a heater.
For this reason, applications of the cold cathode devices have enthusiastically been studied.
Of cold cathode devices, the surface-conduction type emitting device has a simple structure and can be easily manufactured to allow forming many devices on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving many devices has been studied.
Regarding applications of the surface-conduction type emitting devices to, e.g., image forming apparatuses such as image display apparatuses and image recording apparatuses, charge beam sources, and the like have been studied.
Particularly as an application to image display apparatuses, as disclosed in the U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open Nos. 2-257551 and 4-2813.7 filed by the present applicant, an image display apparatus using a combination of surface-conduction type emitting devices and fluorescent substances which emit light upon irradiation with an electron beam has been studied. This type of image display apparatus using a combination of surface-conduction type emitting devices and fluorescent substances is expected to exhibit more excellent characteristics than other conventional image display apparatuses. For example, compared with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require any backlight because of a self-emission type and that it has a wide view angle.
A method of driving many FE type emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. As a known example of an application of FE type emitting devices to an image display apparatus is a flat display apparatus reported by R. Meyer et al. [R. Meyer: “Recent Development on Microtips Display at LETI”, Tech. Digest of 4th Int. Vacuum Microele-ctronics Conf., Nagahama, pp. 6-9 (1991)].
An example of an application of many MIM type emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
The present inventors have examined cold cathode devices of various materials, manufacturing methods, and structures, in addition to the prior arts. Further, the present inventors have made extensive studies on a multi electron beam source having many cold cathode devices, and an image display apparatus using this multi electron beam source.
FIG. 31 shows a multi electron beam source by an electrical wiring method examined by the present inventors. More specifically, this multi electron beam source is constituted by two-dimensionally arranging many cold cathode devices, and wiring these devices in a matrix, as shown in FIG. 31. In FIG. 31, reference numeral 4001 denotes a schematic cold cathode device; 4002, a row-direction wiring line; and 4003, a column-direction wiring line. In practice, the row-direction wiring line 4002 and column-direction wiring line 4003 have finite electrical resistances, which are represented as wiring resistances 4004 and 4005 in FIG. 31. This wiring method is called a simple matrix wiring method.
For the illustrative convenience, the multi electron beam source is illustrated in a 6×6 matrix, but the size of the matrix is not limited to this. For example, in a multi electron beam source for an image display apparatus, devices enough to display a desired image are arranged and wired.
In a multi electron beam source in which cold cathode devices are wired in a simple matrix, appropriate electrical signals are applied to the row-direction wiring line 4002 and column-direction wiring line 4003 in order to output a desired electron beam. For example, to drive cold cathode devices on an arbitrary row in the matrix, a selection voltage Vs is applied to the row-direction wiring line 4002 on the row to be selected, and at the same time a non-selection voltage Vns is applied to the row-direction wiring lines 4002 on unselected rows. In synchronism with this, a driving voltage Ve for outputting an electron beam is applied to the column-direction wiring lines 4003. According to this method, so long as voltage drops across the wiring resistances 4004 and 4005 are neglected, a voltage Ve-Vs is applied to cold cathode devices on the selected row, and a voltage Ve-Vns is applied to cold cathode devices on the unselected rows. If the voltages Ve, Vs, and Vns are set to appropriate levels, an electron beam having a desired intensity must be output from only cold cathode devices on the selected row. If different driving voltages Ve are applied to respective column-direction wiring lines, electron beams having different intensities must be output from respective devices on the selected row. If the application time of the driving voltage Ve is changed, the electron beam output time must be changed.
Hence, a multi electron beam source having cold cathode devices wired in a simple matrix can be applied for variety purposes. For example, if an electrical signal corresponding to image information is properly applied, the multi electron beam source can be preferably used as an electron source for an image display apparatus.
In practice, however, the multi electron beam source having cold cathode devices wired in a simple matrix suffers the following problems.
When the power source of the image display apparatus is turned on, before outputs to be applied from voltage power sources to row-direction wiring lines and column-direction wiring lines stabilize, the outputs from the power sources are applied to the multi electron beam source to damage cold cathode devices.
The same phenomenon occurs when the power source is turned off.
When the potential difference between an acceleration potential for accelerating electrons from the electron source and a potential supplied to the electron source in order to emit electrons is large, and particularly when the potential difference between the electron emission potential and the acceleration potential is 500 V or more, 3 kv or more, or 5 kV or more, an unexpected power source operation may occur while a high acceleration potential is applied. In this case, a discomfort display state may be generated, or the performance of the display panel such as the characteristics of the fluorescent substance may be influenced.
It is an object of an invention according to the present application to improve the display state and reduce damage to the image display apparatus when a power source is turned on, the power source is turned off, an outlet is removed, or power fails.